A meeting in a park can be inexpensive... Weather is often a factor.
Language exchanges in parks can be free of charge for attendees.
It's possible to do a hike or some other thing.
Golden Garden (west of Ballard)
Cal Anderson (capital hill)
Gasworks park (North Lake Union)
South Lake Union (near amazon, don't know the name of the park but this is a good place in summer)
Volunteer Park (north capital hill)
Arboretum (east of capital hill)